Add material color services, plugin DTOs, and tests

Introduce IPC wire-format appearance DTOs (PluginIsolation.Contracts) and clarify they are distinct from the runtime PluginSdk snapshot. Update PluginSdk comments to document the runtime-facing snapshot shape. Change ComponentColorSchemeHelper to use the HostMaterialColorProvider and add an overload that accepts a MaterialColorSnapshot. Add new services and pipelines (MaterialColorService, MaterialSurfaceService, WindowMaterialService, WallpaperColorPipeline) and refactor AppearanceThemeService to depend on MaterialColorService while removing legacy internal implementations. Add multiple unit tests (ComponentColorSchemeHelper, PluginAppearanceBoundary, SettingsCatalogService, WallpaperSettingsPageViewModel) and update localization resources with new material_color and wallpaper keys.

LanMountainDesktop.Tests/SettingsCatalogServiceTests.cs

@@ -0,0 +1,39 @@
+using System.Linq;
+using LanMountainDesktop.PluginSdk;
+using LanMountainDesktop.Services.Settings;
+using Xunit;
+
+namespace LanMountainDesktop.Tests;
+
+public sealed class SettingsCatalogServiceTests
+{
+    [Fact]
+    public void BuiltInAppSectionsIncludeIndependentMaterialColorAndWallpaperEntries()
+    {
+        var catalog = new SettingsCatalogService();
+
+        var sections = catalog.GetSections(SettingsScope.App).ToList();
+
+        Assert.Equal(
+            [
+                "general",
+                "material-color",
+                "appearance",
+                "wallpaper",
+                "about"
+            ],
+            sections.Select(section => section.Id));
+
+        var materialColor = sections.Single(section => section.Id == "material-color");
+        Assert.Equal(SettingsCategories.Appearance, materialColor.Category);
+        Assert.Equal(SettingsScope.App, materialColor.Scope);
+        Assert.Equal("settings.material_color.title", materialColor.TitleLocalizationKey);
+        Assert.Equal("Color", materialColor.IconKey);
+
+        var wallpaper = sections.Single(section => section.Id == "wallpaper");
+        Assert.Equal(SettingsCategories.Appearance, wallpaper.Category);
+        Assert.Equal(SettingsScope.App, wallpaper.Scope);
+        Assert.Equal("settings.wallpaper.title", wallpaper.TitleLocalizationKey);
+        Assert.Equal("Image", wallpaper.IconKey);
+    }
+}